The present invention relates generally to apparatus for drying moist webs or sheets of materials, as these are transported through the final stages of a machine in which they have previously passed through one or more web treatment baths. The apparatus is particularly suitable for photographic film developing machinery.
Machines are known which utilize an extended transport roller system for carrying flexible workpieces, such as photographic films or papers. Typical of such machines are those used to develop photographic films. These machines utilize a series of treatment baths, through which the films are carried in succession by means of transport roller racks extending into these treatment baths. After leaving the last of these baths, the films pass through another transport roller rack within which they are subjected to air flow in order to dry the films before they exit from the developing machine.
In this final transport roller rack, which will be referred to hereafter as the dryer rack, it has been customary to provide a system of air distribution tubes for applying a flow of drying air to the film passing through the dryer rack. These distribution tubes were typically positioned parallel to the transport rollers themselves, and adjacent the gaps between rollers. These tubes had air outlets, or nozzles facing toward the film being transported between the rollers. One or more air blowers were also provided, and these were connected to the inlets of the tubes, usually through manifold ducts leading to the individual distribution tubes. Such a system of air distribution tubes was usually present on both sides of the dryer rack, so as to provide drying of both sides of the film.
Dryer racks of the foregoing types were beset with a variety of problems.
The mechanical construction of such a rack was manifestly complex, and correspondingly costly. Not only did the basic roller rack have to be provided, but so did the elaborate system of air distribution tubes, and supply ducts for these tubes. All this hardware had to be mechanically positioned and structurally supported, and this had to be done with considerble precision, so that the air discharge nozzles would be close to the film, but not interfere with its movement or with the movement of the rollers and of their driving mechanisms.
The constrictions to the drying air supply formed by these ducts and tubes required the use of high pressure blowers, which are expensive to acquire and costly to operate.
These drawbacks became accentuated with increasing width of the dryer rack, since this necessitated longer air distribution tubes, stronger mountings, more powerful blowers, etc. Indeed, those problems tended to impose a ceiling on dryer rack width, thereby limiting the ability of automatic film developing machines to process wide films, or to process large numbers of narrower films simultaneously, side by side.
The air ducts and tubes also created maintenance problems since they had a tendency to clog up with dust during operation. This made it necessary to stop the machine and go through quite complicated cleaning procedures including partial disassembly of the machine to reach the clogged air outlets.
In order to allow room for this placement of the distribution tubes, it was also necessary to position consecutive transport rollers quite far apart. Two disadvantages resulted.
First, this placed limits on the total number of such rollers which could be accommodated in a dryer rack of given over-all length.
The rollers in a drying rack are relied on to pick up some of the moisture from the film being transported by these rollers, thereby contributing to the over-all drying process within the rack. By limiting the number of such rollers which can be accommodated, this part of the drying capability of the rack is undesirably restricted.
Secondly, the far-apart spacing of the rollers made it necessary to provide additional guidance members for the film passing from one roller to the next, to prevent the film from deviating from its intended path, and even becoming caught in the rest of the dryer rack mechanism.
Simply lengthening the dryer rack, and concomitantly increasing the number of transport rollers, is not a satisfactory solution. This would increase the over-all size of the film developing machine, which is undesirable and, in some applications, altogether out of the question. It would also further multiply the complexity stemming from additional air distributing tubes, ducting, film guidance members, and so forth. Also such increase in numbers of transport rollers, if driven in the conventional manner by a train of meshing roller drive gears powered from one end of the train, would quickly become hard to drive, and would be subject to chatter and vibration which could easily damage the film emulsion, which is in a delicate and easily damaged state during passage through the dryer rack.